Method for the heat treatment of a steel reinforcement element for tires

ABSTRACT

The method for the heat treatment of a steel reinforcing element (F) for a tire comprises a transformation of the steel microstructure and in which the temperature of the reinforcing element (F) is reduced during the transformation of the steel microstructure by simultaneously extracting heat from the reinforcing element (F) and supplying heat to the reinforcing element (F).

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC 371 ofInternational Application PCT/EP2015/053457 filed on Feb. 19, 2015.

This application claims the priority of French application no. 1451380filed Feb. 21, 2014, the entire content of which is hereby incorporatedby reference.

FIELD OF THE INVENTION

The invention relates to a method for the heat treatment of a steelreinforcing element for a tire.

BACKGROUND OF THE INVENTION

A method is known from the prior art for manufacturing a steelreinforcing element for a tire, for example a steel wire.

The manufacturing method makes it possible to manufacture, from a wire,referred to as wire stock, having an initial diameter between 4.5 and7.5 mm, a wire that can be used for reinforcing plies of the tire havinga diameter of between 0.08 mm and 0.50 mm.

Firstly, the wire stock of predominantly pearlitic microstructure isdrawn, for example in a dry environment, so as to reduce its initialdiameter to an intermediate diameter, for example equal to 1.3 mm. Atthe end of this drawing step, the steel of the wire has a microstructurecomprising several mixed phases.

Next, the wire of intermediate diameter is heat treated so as to modifythe microstructure of the steel. In this instance, the predominantlypearlitic microstructure of the steel is regenerated.

After having coated the wire of intermediate diameter with a metallayer, the coated wire of intermediate diameter is drawn, for example ina wet environment, so as to reduce its diameter to a final diameter, forexample equal to 0.20 mm.

A method for the heat treatment of the wire of intermediate diameter isknown from U.S. Pat. No. 4,767,472 that comprises three steps and iscarried out by means of a heat treatment facility.

The heat treatment facility comprises, in the run direction of the wire,upstream means for storing the untreated wire, for example upstreamreels, a heating device, a cooling device, and downstream means forstoring the treated wire, for example downstream reels.

During a first step, the temperature of the wire is increased above theaustenitizing temperature of the steel in order to obtain apredominantly austenitic microstructure. For this purpose, the heattreatment facility comprises a device for heating the wire comprising agas-fired furnace.

Then, in a second step, the temperature of the wire is reduced in orderto obtain a metastable austenitic microstructure by means of a coolingdevice comprising a water bath. The bath comprises pure liquid water ata temperature above 80° C. through which the wire is made to run.

In a third step, carried out downstream of the bath, the temperature ofthe wires is left to drop in ambient air or else in a thermallyinsulated device. During this exposure to the ambient air, thepredominantly austenitic microstructure is transformed to apredominantly pearlitic microstructure or else this transformation,pre-initiated in the bath, is continued by passing through the pearlitetransformation range.

However, during the transformation, it is not possible to preciselycontrol the rate of temperature reduction, in particular in order tocontrol the recalescence. In order to resolve this problem, documentU.S. Pat. No. 6,228,188 proposes successive passes of the wire throughwater baths alternating with passes through air during thetransformation.

However, this process is relatively tedious to control. Indeed,depending on the desired rate of temperature reduction, it is necessaryto control numerous parameters such as the length of the baths, thelength of the passes through air, the water temperature of the baths,the temperature of the ambient air, the run speed of the wire. Theseparameters must be changed as soon as the nature of the wire varies, inparticular its diameter and the composition of the steel.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a more flexibleheat treatment method.

This and other objects are attained in accordance with one aspect of thepresent invention directed to a method for the heat treatment of a steelreinforcing element for a tire comprising a transformation of the steelmicrostructure and in which the temperature of the reinforcing elementis reduced during the transformation of the steel microstructure bysimultaneously extracting heat from the reinforcing element andsupplying heat to the reinforcing element.

The method according to the invention is easy to control. It makes itpossible to change the nature of the reinforcing element without havingto modify too many parameters. Indeed, the temperature reduction can beeasily controlled by determining the amount of heat that has to beextracted from the reinforcing element. This amount can then be easilyadjusted by simultaneously controlling the extraction and the supply ofthe heat, unlike the method from the prior art in which heat is onlyextracted by means of the baths and the ambient air.

The invention applies to any type of method in which the temperature ofthe reinforcing element is reduced during the transformation of thesteel microstructure and more advantageously to a continuous coolingheat treatment (abbreviated to “CCHT”) method. Unlike an isothermal heattreatment (abbreviated to “IHT”) method that uses a TTT(time-temperature-transformation) diagram and comprises one or morechanges of rate during the temperature reduction step, a CCHT-typemethod uses a CCT (continuous cooling transformation) diagram and has acontinuous rate during the temperature reduction step.

In the present description, any range of values denoted by theexpression “between a and b” represents the field of values ranging frommore than a to less than b (that is to say limits a and b excluded)whereas any range of values denoted by the expression “from a to b”means the field of values ranging from a up to b (that is to sayincluding the strict limits a and b).

In one embodiment, the heat is extracted from the reinforcing element bythermal convection in contact with at least one cold source.

In one embodiment, heat is supplied to the reinforcing element by theJoule effect through the reinforcing element.

Supplying heat by the Joule effect allows a direct supply of heat to thereinforcing element through the latter. Thus, it is possible to increasethe temperature very rapidly which makes it possible to use a high runspeed and therefore to obtain a high unit mass throughput. Indeed,supplying heat by the Joule effect is extremely effective since it iscarried out through the wire without convection.

In addition, supplying heat by the Joule effect leads to a relativelylow and controlled energy expenditure relative to other means forsupplying heat, in particular of the convection type such as gas-firedfurnaces.

Preferably, the heat is extracted from the reinforcing element by virtueof means for extracting heat from the reinforcing element comprising:

a run chamber of the reinforcing element containing an intermediate coldsource arranged between the reinforcing element and an external coldsource,

a chamber for circulation of the external cold source arranged aroundthe run chamber of the reinforcing element.

Such extraction means are compatible with a high run speed. Indeed, theheat extraction means have a heat-extraction capacity that issubstantially greater than the water bath used in the prior art whichmakes it possible to increase the run speed of the reinforcing element.

Furthermore, a high speed in the bath used in the prior art leads to aturbulent flow of the water in contact with the wire and therefore to aninsufficient and poorly controlled reduction in the temperature of thewire, then leading to the appearance of surface defects of the wire. Byeliminating the water bath, the problem of the appearance of turbulentflow at high speed, and therefore the problem of the appearance ofsurface defects of the reinforcing element, are eliminated.

In addition, the extraction means are much safer than the bathscustomarily used, whether they are water baths, lead baths or moltensalt baths. Indeed, the circulation chamber makes it possible tophysically isolate the reinforcing element from the operators.

Unlike the lead or molten salt baths that may pose environmental andsafety problems, the extraction means are safe and environmentallyfriendly. Furthermore, the heat extraction means used make it possibleto avoid any cleaning step that aims to eliminate the lead or the moltensalts covering the reinforcing element.

In one embodiment, the intermediate cold source comprises a heatexchange gas. In one embodiment, the heat exchange gas may comprise oneor more gaseous constituents.

Advantageously, heat exchange gas comprises a gas selected from reducinggases, inert gases and mixtures of these gases, preferably from reducinggases, and more preferentially is dihydrogen.

In one embodiment, the external cold source comprises a heat exchangeliquid.

Preferably, heat is supplied to the reinforcing element by virtue ofmeans for supplying heat by the Joule effect through the reinforcingelement comprising two electrically conductive terminals.

Advantageously, each electrically conductive terminal comprises anelectrically conductive rotatable pulley.

The rotatable pulleys allow both the electrical conduction and also thepassage and guiding of the reinforcing element, irrespective of the runspeed of the latter.

Advantageously, the reinforcing element is made to run at a mean runspeed strictly greater than 40 m·min⁻¹, preferably strictly greater than90 m·min⁻¹, more preferentially greater than or equal to 200 m·min⁻¹ andmore preferentially still greater than or equal to 300 m·min⁻¹.

The mean speed should be understood to mean the ratio of the distancetravelled by one point of the reinforcing element to the time taken bythis point to travel this distance.

Unlike the method described in U.S. Pat. No. 4,767,472 and the IHTmethods that inevitably require a relatively long transformation time,for example of the order of several tens of seconds, the transformationin a CCHT-type method may be relatively short, for example of the orderof several seconds, which makes it possible to use high run speeds in afacility having a reduced size.

In one embodiment, the mean rate of temperature reduction during thetransformation of the microstructure of the steel is greater than orequal to 30° C.s⁻¹, preferably greater than or equal to 50° C.s⁻¹ andmore preferentially greater than or equal to 70° C.s⁻¹.

The use of too low a mean rate of reduction does not make it possible torapidly carry out the transformation of the steel microstructure. Thus,the risk of obtaining a steel having undesired mechanical properties isminimized.

In one embodiment, the mean rate of temperature reduction during thetransformation of the microstructure of the steel is less than or equalto 110° C.s⁻¹, preferably less than or equal to 100° C.s⁻¹ and morepreferentially less than or equal to 90° C.s⁻¹.

The use of too high a rate of reduction has the risk of resulting in aquenching of the steel which according to the desired properties of thesteel is not desirable.

The mean rate of reduction is understood to mean the ratio of thedifference in degrees Celsius between the temperature beforetransformation and after transformation to the time taken to carry outthe transformation.

Thus, several embodiments could be envisaged in which the mean rate ofreduction is within ranges extending from 30° C.s⁻¹ to 90° C.s⁻¹, from30° C.s⁻¹ to 100° C.s⁻¹, from 30° C.s⁻¹ to 110° C.s⁻¹, from 50° C.s⁻¹ to90° C.s⁻¹, from 50° C.s⁻¹ to 100° C.s⁻, from 50° C.s⁻¹ to 110° C.s⁻¹,from 70° C.s⁻¹ to 90° C.s⁻¹, from 70° C.s⁻¹ to 100° C.s⁻¹ and from 70°C.s⁻¹ to 110° C.s⁻¹.

In one embodiment, the temperature is reduced by more than 30° C.,preferably by more than 50° C., more preferentially by more than 75° C.and more preferentially still by more than 100° C. during thetransformation of the steel microstructure.

In one embodiment, the transformation of the steel microstructure takesplace in a temperature range extending from 800° C. to 400° C.,preferably from 750° C. to 500° C. and more preferentially from 650° C.to 550° C.

The expression “by more than X° C.” means that the temperature isreduced by a temperature range strictly greater than X° C., the value ofX° C. therefore being excluded.

Preferably, the method comprises a step of reducing the temperature ofthe reinforcing element by continuous cooling:

from an initial temperature of an initial stability range of the steel,

to a final temperature of a final stability range of the steel,

the temperature reduction step comprising a transformation of the steelmicrostructure from a microstructure of the initial range to amicrostructure of the final range.

The method is thus relatively robust and simple to control. Indeed,unlike the method described in U.S. Pat. No. 4,767,472 and certain heattreatment methods from the prior art, referred to as isothermaltransformation methods (abbreviated to “IHT” for isothermal heattreatment), in which the transformation of the steel takes place at asubstantially constant temperature, the method here involves continuouscooling (abbreviated to “CCHT” for continuous cooling heat treatment).The two types of methods are easily distinguished, in particular bymeans of the time-temperature diagrams used to represent them. AnIHT-type method uses a TTT (time-temperature-transformation) diagram andcomprises one or more changes of rate during the temperature reductionstep. A CCHT-type method uses a CCT (continuous cooling transformation)diagram and has a continuous rate during the temperature reduction step.Thus, among other features of a CCHT-type method, the temperature of thereinforcing element is reduced during the transformation of the steelmicrostructure.

In the method described above, once the rate of temperature reduction isdefined, it is then relatively easy to control it considering itscontinuity during the temperature reduction step.

In addition, unlike the IHT-type methods in which a very large amount ofheat is supplied to the reinforcing element in order to keep it atsubstantially constant temperature during the transformation, the methoddescribed being of CCHT type makes it possible to reduce the energyconsumption of the method.

In one embodiment, the initial temperature is greater than or equal to750° C., preferably greater than or equal to 800° C. and morepreferentially greater than or equal to 850° C.

In one embodiment, the final temperature is less than or equal to 650°C., preferably less than or equal to 550° C. and more preferentiallyless than or equal to 450° C.

Thus, several embodiments could be envisaged in which the initialtemperature/final temperature pairs are 750° C./450° C., 750° C./550°C., 750° C./650° C., 800° C./450° C., 800° C./550° C., 800° C./650° C.,850° C./450° C., 850° C./550° C., 850° C./650° C.

In one embodiment, the initial range is the austenite stability range ofthe steel. Thus, the steel preferentially has a predominantly austeniticinitial microstructure.

In one embodiment, the final range is the ferrite-pearlite stabilityrange of the steel. Thus, in this ferrite-pearlite stability range, thesteel has a predominantly ferritic-pearlitic final microstructure. Thesteel may also comprise, depending on its composition and on the variousparameters of the method, bainite and/or ferrite and/or martensite. Inthe case where one or more of these microstructures is present, theferritic-pearlitic microstructure is preferably predominant.

A predominantly austenitic/ferritic-pearlitic microstructure isunderstood to mean, in a manner known to a person skilled in the art,that the steel comprises a predominant austenitic/pearlitic phaserelative to the other phases that make up the steel. A predominant phaseis understood to mean that the steel comprises, by weight, a higherpercentage of this phase relative to the sum of the percentages of theother phases, i.e. more than 50%, preferably more than 75%, or even morethan 95% and more preferentially more than 98% by weight of this phaserelative to the sum of the percentages of the other phases.

Advantageously, the temperature reduction step comprises a reduction inthe temperature of the reinforcing element in the initial stabilityrange of the steel.

Advantageously, the temperature reduction step comprises a reduction inthe temperature of the reinforcing element in the final stability rangeof the steel.

In one embodiment, the steel microstructure is transformed by passingthrough at least one transformation range.

Advantageously, the temperature for entering the transformation range,i.e. the temperature delimiting the passage between the initialstability range and the transformation range, is greater than or equalto 550° C., preferably greater than or equal to 600° C., morepreferentially greater than or equal to 650° C. and more preferentiallystill greater than or equal to 700° C.

Advantageously, the temperature for leaving the transformation range,i.e. the temperature delimiting the passage between the transformationrange and the final stability range, is greater than or equal to 400°C., preferably greater than or equal to 500° C., more preferentiallygreater than or equal to 600° C. and more preferentially still greaterthan or equal to 650° C.

In one preferred embodiment, the transformation range comprises theferrite transformation range.

In one preferred embodiment, the transformation range comprises thepearlite transformation range.

In one embodiment, prior to the step of reducing the temperature of thereinforcing element, the method comprises a step of increasing thetemperature of the reinforcing element to a temperature greater than orequal to the austenitizing temperature of the steel.

By heating the steel above the austenitizing temperature of the steel, apredominantly austenitic, or even completely austenitic microstructureis obtained.

Optionally, heat is supplied to the reinforcing element during at leastone portion of the step of reducing the temperature of the reinforcingelement.

The supply of heat to the reinforcing element makes it possible tocontrol the reduction in temperature of the reinforcing element andtherefore to obtain the desired microstructure of the steel. Inparticular, the supply of heat makes it possible not to reduce thetemperature too rapidly which would result in the creation of undesiredphases in the microstructure of the steel.

Unlike the supply of heat of an IHT-type method, the supply of heat ofthe method is relatively small since it does not have the objective ofkeeping the temperature of the reinforcing element constant.

According to other optional features of the method:

-   -   The steel reinforcing element is a steel wire.    -   The steel wire has a diameter ranging from 0.5 to 5.5 mm,        preferably from 0.5 to 3 mm and more preferentially from 1 to        2.5 mm.    -   The steel comprises from 0.4% to 1.2%, preferably from 0.4% to        1% and more preferentially from 0.4% to 0.8% of carbon by        weight.

Another subject of the invention is a steel reinforcing element capableof being obtained by a method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from reading the followingdescription, given solely by way of non-limiting example and withreference to the drawings in which:

FIG. 1 is a diagram of a heat treatment facility according to a firstembodiment for the implementation of the method according to anembodiment of the invention;

FIG. 2 is a diagram of a device for heating the facility in FIG. 1;

FIG. 3 is a diagram of a temperature-maintaining device of the facilityin FIG. 1;

FIG. 4 is a diagram of a device for cooling the facility in FIG. 1;

FIG. 5 is a cross-sectional view along V-V of the cooling device in FIG.4;

FIG. 6 is a diagram illustrating various steps of a method formanufacturing a reinforcing element comprising steps of a heat treatmentmethod according to an embodiment of the invention;

FIG. 7 is a CCT time-temperature diagram illustrating the heat treatmentmethod of FIG. 6; and

FIG. 8 is a diagram of a heat treatment facility according to a secondembodiment.

DETAILED DESCRIPTION OF THE DRAWINGS

Example of a Heat Treatment Facility for the Implementation of theMethod According to the Invention

Represented in FIG. 1 is a first embodiment of a facility for the heattreatment of a reinforcing element for a tire, denoted by the generalreference 10.

The treatment facility 10 is capable of treating steel reinforcingelements F, here steel wires. The steel wires F have a diameter rangingfrom 0.5 to 5.5 mm, preferably from 0.7 to 3 mm and more preferentiallyfrom 1 to 2.5 mm.

The facility 10 comprises, in the run direction of the element F in thefacility 10, from upstream to downstream, upstream means 12 for storingthe element F, a device 14 for heating the element F, a device 15 formaintaining the element F at temperature, a device 16 for cooling theelement F and downstream means 18 for storing the heat-treated elementF.

The upstream 12 and downstream 18 storage means each comprise a reel forstoring the element F respectively making it possible to unwind and windup the element F. In the upstream storage means 12, the element F isthen connected to the potential P0.

The heating device 14 is represented in FIG. 2. The device 14 makes itpossible to heat the element F at a temperature greater than or equal tothe austenitizing temperature of the steel.

The heating device 14 comprises means 20 for supplying heat to theelement F. The heat supply means 20 comprise means 21 for supplying heatby the Joule effect through the element F. These heat supply means 21comprise two electrically conductive terminals 22, 24 powered by acurrent source, here a transformer 26. Each terminal 22, 24 isrespectively connected to the phase conductor P1 and the neutralconductor N1. In this instance, each terminal 22, 24 respectivelycomprises an electrically conductive rotatable pulley 23, 25. Eachterminal 22, 24 is arranged so that each pulley 23, 25 is in contactwith the element F during the operation of the facility 10.

The temperature-maintaining device 15 is represented in FIG. 3. Thedevice 15 is arranged between the heating device 14 and the coolingdevice 16 and makes it possible to maintain the temperature of theelement F at a temperature greater than or equal to the austenitizingtemperature of the steel.

The maintaining device 15 comprises means 31 for supplying heat to theelement F. The heat supply means 31 comprise means 33 for supplying heatby the Joule effect through the element F. These heat supply means 31comprise two electrically conductive terminals 35, 37 powered by acurrent source, here a transformer 39. Each terminal 35, 37 isrespectively connected to the phase conductor P2 and the neutralconductor N2. In this instance, each terminal 35, 37 respectivelycomprises an electrically conductive rotatable pulley 41, 43. Eachterminal 35, 37 is arranged so that each pulley 41, 43 is in contactwith the element F during the operation of the facility 10.

The facility 10 also comprises means 49 for applying the element Fagainst the terminals 24, 35 and more specifically in contact with thepulleys 25, 41. The application means 49 here comprise a rotatablepulley 51.

The cooling device 16 is represented in FIGS. 4 and 5.

The cooling device 16 comprises means 30 for supplying heat to theelement F and means 32 for extracting heat from the element F to atleast one cold source, here two cold sources. The cold source(s) aredifferent from ambient air.

The heat supply means 30 comprise means 34 for supplying heat by theJoule effect through the element F. The heat supply means 34 comprisetwo electrically conductive terminals 36, 38 respectively positionedupstream and downstream of an inlet 40 and an outlet 42 of the element Finto/from the cooling device 16. Each terminal 36, 38 is respectivelyconnected to the phase conductor P3 and the neutral conductor N3, theneutral conductor N3 being at the same potential as the earth T. Eachterminal 36, 38 respectively comprises an electrically conductiverotatable pulley 53, 55. The heat supply means 34 also comprise acurrent source, here a transformer 44, powering the two terminals 36,38. Each terminal 36, 38 is arranged so that each pulley 53, 55 is incontact with the element F during the operation of the facility 10.

The facility 10 also comprises means 61 for applying the element Fagainst the terminals 37, 36 and more specifically in contact with thepulleys 43, 53. The application means 61 here comprise a rotatablepulley 63.

The heat extraction means 32 are of the type having convective exchangebetween the element F and the cold source(s).

The heat extraction means 32 comprise a run chamber 46 of the element F.The run chamber 46 forms a jacket and has a general shape with axialsymmetry with respect to the run axis X of the element F, in thisinstance having a circular general cross section. The run chamber 46contains an intermediate cold source 48. The heat exchange between theelement F and the intermediate cold source 48 occurs by convection, hereby forced convection due to the circulation of the intermediate coldsource 48 in the run chamber 46 from upstream to downstream in the rundirection of the element F. As a variant, the cold source is devoid offorced convection. For example, the intermediate cold source 48comprises a heat exchange gas 50. Preferably, the gas 50 is selectedfrom reducing gases, inert gases and mixtures of these gases. Morepreferentially, the gas 50 is a reducing gas, here dihydrogen H₂. As avariant, the gas 50 comprises several gaseous constituents, for examplean H₂+N₂ mixture. In another variant, the gas 50 is nitrogen N₂.

The heat extraction means 32 also comprise a chamber 52 for circulationof an external cold source 54. The circulation chamber 52 forms a jacketand has a general shape with axial symmetry with respect to the run axisX of the element F, in this instance having a circular general crosssection. The circulation chamber 52 is radially external with respect tothe run chamber 46 and arranged around the latter. The intermediate coldsource 48 is arranged between the element F and the external cold source54. The heat exchange between the intermediate cold source 48 and theexternal cold source 54 occurs by convection, here by forced convectiondue to the circulation of the external cold source 54 in the circulationchamber 52, from downstream to upstream in the run direction of theelement F.

For example, the external cold source 54 comprises a heat exchangeliquid 56, here water.

The heat supply means 30 and heat extraction means 32 are arranged sothat the temperature of the element F at the outlet 42 is strictly lessthan the temperature of the element F at the inlet 40.

Example of a Heat Treatment Method According to the Invention

An example of a method for the heat treatment of the steel reinforcingelement F for a tire will now be described with reference to FIGS. 6 and7 within the context of a method for manufacturing the element F.

The steel comprises for example from 0.4% to 1.2%, preferably from 0.4%to 1% and more preferentially from 0.4% to 0.8% of carbon by weight. Thesteel may also comprise specific alloying elements such as Cr, Ni, Co,V, or various other known elements (see, for example, ResearchDisclosure 34984—“Micro-alloyed steel cord constructions for tires”—May1993; Research Disclosure 34054—“High tensile strength steel cordconstructions for tires”—August 1992). In this instance, a conventionalsteel containing 0.7% of carbon is used.

Prior to the heat treatment method, the element F is drawn, for examplein a dry environment, so as to reduce its initial diameter, equal to 5.5mm, to an intermediate diameter, here equal to 1.3 mm, in a step 100. Atthe end of this drawing step 100, the steel of the element F has amicrostructure comprising several mixed phases.

The heat treatment method is then carried out in which the element F ofintermediate diameter is heat treated so as to modify the microstructureof the steel. In this instance, the predominantly pearliticmicrostructure is regenerated.

The heat treatment method comprises a step 200 of increasing thetemperature of the element F from a temperature T0 to a temperature T1greater than or equal to the austenitizing temperature of the steel. Asillustrated in FIG. 7, this step lasts from t0 to t1 and is carried outby means of the heating device 14.

During at least one part of this step 200, heat is provided to theelement F by the Joule effect through the element F. Each terminal 22,24 is in contact with the element F during this step 200.

The temperature of the element F is increased from T0 to T1 at a meanrate of increase ranging from 100 to 1000° C.s⁻¹, preferably 500 to 950°C.s⁻¹ and more preferentially from 700 to 900° C.s⁻¹. Here, the rate ofincrease is equal to 836° C.s⁻¹.

Preferably, T0 is less than or equal to 100° C. and more preferentiallyless than or equal to 50° C. Here, T0=20° C. Preferably, T1 is greaterthan or equal to 850° C. and more preferentially greater than or equalto 900° C. Here, T1=975° C.

The method comprises a step 202 of maintaining the temperature of theelement F at a temperature greater than or equal to the temperature T1.As illustrated in FIG. 7, this step 202 lasts from t1 to t2 and iscarried out by means of the temperature-maintaining device 15.

As a variant, the method may not comprise a step 202 of maintaining thetemperature of the steel at the austenitizing temperature of the steel.Thus, a method could be envisaged without a temperature-maintainingstep, that is to say in which the step of increasing the temperature ofthe element F from a temperature T0 to a temperature T1 greater than orequal to the austenitizing temperature of the steel and the step ofreducing the temperature described below are carried out consecutively.In this variant, the facility 10 does not comprise a device 15 arrangedbetween the heating device 14 and the cooling device 16.

Then, the element F arrives at the inlet 40 of the heat extraction means32 of the device 16 at the time t2. The method then comprises a step 204of reducing the temperature of the element F by continuous cooling froman initial temperature T2 to a final temperature T3. As illustrated inFIG. 7, this step 204 lasts from t2 to t3 and is carried out by means ofthe cooling device 16.

In order to reduce the temperature from T2 to T3, heat is extracted fromthe element F by thermal convection in contact with the intermediatecold source 48, in the example by virtue of the heat extraction means32.

The temperature is reduced from T2 to T3 at a mean rate of reductiongreater than or equal to 30° C.s⁻¹, preferably greater than or equal to50° C.s⁻¹ and more preferentially greater than or equal to 70° C.s⁻¹. Inthis preferred embodiment, the rate of reduction is less than or equalto 110° C.s⁻¹, preferably less than or equal to 100° C.s⁻¹ and morepreferentially less than or equal to 90° C.s⁻¹.

In the example described, T2 is greater than or equal to 750° C.,preferably greater than or equal to 800° C. and more preferentiallygreater than or equal to 850° C. Here, T2=T1=975° C.

In the example described, T3 is less than or equal to 650° C.,preferably less than or equal to 550° C. and more preferentially lessthan or equal to 450° C. Here, T3=400° C.

Preferably, heat is supplied to the element F during at least one partof the reduction step 204, in the example by virtue of the means 34 forsupplying heat by the Joule effect through the element F which is thenin contact with the terminals 36, 38.

The temperature of the reinforcing element F is strictly decreasingduring the reduction step 204.

The temperature reduction step 204 is after the temperature-maintainingstep 202 which is itself after the temperature increase step 200.

The element F is made to run at a mean run speed preferably strictlygreater than 40 m·min⁻¹, preferably strictly greater than 90 m·min⁻¹,more preferentially greater than or equal to 200 m·min⁻¹ and morepreferentially still greater than or equal to 300 m·min⁻¹. Here, the runspeed is equal to 315 m·min⁻¹.

The inlet temperature of the water is between 20° C. and 40° C., andhere substantially equal to 15° C. The inlet temperature of the gas issubstantially equal to 20° C.

The element F then arrives at the outlet 42 of the heat extraction means32 of the device 16 at the time t3. The element F obtained by the methodhas, in this example, a tensile strength Rm equal to 1150 MPa.

After the heat treatment method, in a step 300, the heat-treated elementF of intermediate diameter is coated with a metal layer, for example alayer of brass.

Then, in a step 400, the coated, heat-treated element F of intermediatediameter is drawn, for example, in a wet environment so as to reduce itsdiameter to a final diameter, for example equal to 0.23 mm.

The element F thus obtained could be used as an individual wire forreinforcing tire plies or else for the manufacture of a layered cord orelse a stranded cord for reinforcing tire plies.

The heat treatment method has been illustrated in FIG. 7 by a curve Crepresenting the change in the temperature of the element F as afunction of the time. The reduction step 204 will be described withreference to FIG. 7.

The initial temperature T2 belongs to an initial stability range of thesteel, here the austenite stability range I in which the steel has apredominantly austenitic microstructure.

The final temperature T3 belongs to a final stability range of thesteel, here the pearlite stability range IV in which the steel has apredominantly pearlitic microstructure.

The reduction step 204 comprises a reduction of the temperature of theelement F in the initial stability range, here in the austenitestability range. This reduction is illustrated by the portion C1 of thecurve C between the points (t2, T2) and (t2′, T2′).

Next, the reduction step 204 comprises a transformation of the steelmicrostructure from the microstructure of the initial range, hereaustenitic, to a microstructure of the final range, hereferritic-pearlitic. The steel microstructure is transformed by passingthrough at least one transformation range. In this instance, the ferritetransformation range II (portion C2 of the curve C) and pearlitetransformation range III (portion C3 of the curve C) are passed throughsuccessively between the points (t2′, T2′) and (t3′, T3′). Thetransformation ranges of the steel are distinct from the bainite rangeand preferably from the martensite range.

This transformation of the steel microstructure takes place in atemperature range [T2′, T3′] extending from 800° C. to 400° C.,preferably from 750° C. to 500° C. and more preferentially from 650° C.to 550° C. The temperature T2′ for entering the ferrite transformationrange II, i.e. the temperature delimiting the passage between theinitial stability range I and ferrite transformation range II, isgreater than or equal to 550° C., preferably greater than or equal to600° C., more preferentially greater than or equal to 650° C. and morepreferentially still greater than or equal to 700° C. The temperatureT3′ leaving the pearlite transformation range III, i.e. the temperaturedelimiting the passage between the pearlite transformation range III andthe final stability range IV, is greater than or equal to 400° C.,preferably greater than or equal to 500° C., more preferentially greaterthan or equal to 600° C. and more preferentially still greater than orequal to 650° C. In this instance, T2′=710° C. and T3′=600° C.

During this transformation of the steel microstructure, the temperatureof the element F is reduced, by simultaneously extracting heat from theelement F by thermal convection in contact with the intermediate coldsource 48 and supplying heat to the element F by the Joule effectthrough the element F. In order to reduce the temperature of the elementF, more heat is extracted than is supplied thereto.

During this transformation of the steel microstructure, the temperatureof the element F is reduced for example by more than 30° C., preferablyby more than 50° C., more preferentially by more than 75° C. and morepreferentially still by more than 100° C. In this instance, thetemperature is reduced by 123° C.

During this transformation, the mean rate of temperature reduction isgreater than or equal to 30° C.s⁻¹, preferably greater than or equal to50° C.s⁻¹ and more preferentially greater than or equal to 70° C.s⁻¹.

During this transformation, the mean rate of temperature reduction isless than or equal to 110° C.s⁻¹, preferably less than or equal to 100°C.s⁻¹ and more preferentially less than or equal to 90° C.s⁻¹.

In this instance, the mean rate of temperature reduction is equal to 86°C.s⁻¹.

Next, the reduction step 204 comprises a reduction of the temperature ofthe element F in the final stability range, here in the pearlitestability range. This reduction is illustrated by the portion C4 of thecurve C between the points (t3′, T3′) and (t3, T3).

It will be noted that the method described here is a continuous coolingmethod. Thus, as illustrated in the CCT diagram of FIG. 7, there is nosudden change in the rate of cooling of the element F between thetemperatures T2 and T3. In addition, the transformation takes placecompletely between the inlet 40 and the outlet 42 of the cooling device16. In other words, the element F leaves the initial stability rangedownstream of the inlet 40 and upstream of the outlet 42 and reaches thefinal stability range downstream of the inlet 40 and upstream of theoutlet 42.

By transforming the steel microstructure between the inlet 40 and theoutlet 42 of the cooling device 16, the formation of oxides at thesurface of the steel is limited, or even avoided, unlike a heattreatment that takes place entirely or partly in an oxidizing medium,for example in ambient air.

A person skilled in the art is capable of distinguishing the austenitic,pearlitic, bainitic, ferritic and martensitic microstructures describedabove, in particular by microscopic observation by known means, forexample a scanning electron microscope (SEM) or electron backscatterdiffraction (EBSD). This observation could, in a known manner, bepreceded by a chemical etching.

The features relating to the operation of the facility 14, such as theintensity of the current in the devices 14 and 16, the temperature ofthe intermediate cold source 48 and external cold source 54, the flowrate of the external cold source 54, are in particular a function of thesize of the element F, of its diameter in the case of a wire, and of therun speed of the element F. The values of these features are within thecapabilities of a person skilled in the art who will be able todetermine them by successive tests or else by calculation.

Represented in FIG. 8 is a second embodiment of a facility for the heattreatment of a reinforcing element for a tire. Elements similar to thoserepresented in the first embodiment are denoted by identical references.

Unlike the first embodiment, the facility according to the secondembodiment comprises several cooling devices 16 arranged in seriesdownstream of the heating device 14.

The cooling devices 16 are connected together by ducts 58 forcirculation of the element F. These ducts also enable the circulation ofthe intermediate cold source 48. The cooling devices 16 are alsoconnected together by ducts 60 for circulation of the external coldsource 54.

The invention is not limited to the embodiments described above.

In particular it could be envisaged to heat-treat several reinforcingelements simultaneously in one and the same cooling device. Thus,several reinforcing elements may run through the heat extraction meanswhich makes it possible to further increase the total mass throughputwithout increasing the size of the heat treatment facility.

It could also be envisaged to superimpose several heat treatmentfacilities on top of one another in order to reduce the floor spacerequirement.

The reinforcing element could be different from a wire, in particulardifferent from a wire of circular cross section.

The heat treatment method according to the invention could be carriedout within the context of a manufacturing method different from thatdescribed above. In particular, a manufacturing method comprising twodry-drawing steps or two wet-drawing steps could be used.

It may also be possible to combine the features of the variousembodiments described or envisaged above, as long as these arecompatible with one another.

The great ease of operation of the method and also the reduced size ofthe facility described above will be noted, unlike the method andfacility from U.S. Pat. No. 4,767,472 in which the relatively low valueof the run speed of the facility is compensated for by the simultaneoustreatment of several tens of wires that makes it possible to obtain ahigh total mass throughput despite a mass throughput per wire, i.e. aunit mass throughput, that is low. Furthermore, in U.S. Pat. No.4,767,472, when the upstream storage reels are empty, it is necessary tojoin the end of each wire to another wire originating from a full reel.It is therefore necessary to carry out as many joining operations asthere are wires, which makes the operation of the heat treatmentfacility from U.S. Pat. No. 4,767,472 relatively complex and tedious. Inaddition, it is necessary to have as many reels as there are wires,which makes the heat treatment facility from U.S. Pat. No. 4,767,472relatively large.

The scope of protection of the invention is not limited to the examplesgiven hereinabove. The invention is embodied in each novelcharacteristic and each combination of characteristics, which includesevery combination of any features which are stated in the claims, evenif this feature or combination of features is not explicitly stated inthe examples.

The invention claimed is:
 1. A method for the heat treatment of a steelreinforcing element for a tire, comprising: transforming the steelmicrostructure, said transforming comprising a step of: reducing atemperature of the reinforcing element during the transformation of thesteel microstructure in a cooling device having a heat extractor, theheat extractor having an inlet (40) at an entry end of the heatextractor, and an outlet (42) located at an exit end of the heatextractor, the cooling device further having two electrically conductiveterminals (36, 38), respectively positioned upstream and downstream ofthe inlet (40) and the outlet (42), by: extracting heat from thereinforcing element within the cooling device by the heat extractorarranged within the cooling device while simultaneously supplying heatto the reinforcing element within the cooling device by application ofJoule effect heating to the reinforcing element between the twoelectrically conductive terminals (36, 38).
 2. The method according toclaim 1, wherein the heat is extracted from the reinforcing elementwithin the cooling device by thermal convection in contact with at leastone cold source.
 3. The method according to claim 2, wherein the heat isextracted from the reinforcing element within the cooling device by: arun chamber containing an intermediate cold source arranged between thereinforcing element and an external cold source, and a chamber forcirculation of the external cold source arranged around the run chamberof the reinforcing element.
 4. The method according to claim 1, whereineach of the two electrically conductive terminals comprises anelectrically conductive rotatable pulley.
 5. The method according toclaim 1, wherein the reinforcing element has a mean run speed greaterthan 40 m·min⁻¹.
 6. The method according to claim 1, comprising movingthe reinforcing element at a mean run speed greater than 90 m·min⁻¹while reducing the temperature of the reinforcing element.
 7. The methodaccording to claim 1, comprising moving the reinforcing element at amean run speed greater than or equal to 200 m·min⁻¹ while reducing thetemperature of the reinforcing element.
 8. The method according to claim1, comprising moving the reinforcing element at a mean run speed greaterthan or equal to 300 m·min⁻¹ while reducing the temperature of thereinforcing element.
 9. The method according to claim 1, wherein themean rate of temperature reduction during the transformation of themicrostructure of the steel is greater than or equal to 30° C.s⁻¹. 10.The method according to claim 1, wherein the mean rate of temperaturereduction during the transformation of the microstructure of the steelis greater than or equal to 50° C.s⁻¹.
 11. The method according to claim1, wherein the mean rate of temperature reduction during thetransformation of the microstructure of the steel is greater than orequal to 70° C.s⁻¹.
 12. The method according to claim 1, wherein themean rate of temperature reduction during the transformation of themicrostructure of the steel is less than or equal to 110° C.s⁻¹.
 13. Themethod according to claim 1, wherein the temperature is reduced by morethan 30° C. during the transformation of the steel microstructure. 14.The method according to claim 1, wherein the temperature is reduced bymore than 50° C. during the transformation of the steel microstructure.15. The method according to claim 1, wherein the temperature is reducedby more than 75° C. during the transformation of the steelmicrostructure.
 16. The method according to claim 1, wherein thetemperature is reduced by more than 100° C. during the transformation ofthe steel microstructure.
 17. The method according to claim 1, whereinthe transformation of the steel microstructure takes place in atemperature range from 800° C. to 400° C.
 18. The method according toclaim 1, wherein the transformation of the steel microstructure takesplace in a temperature range from 750° C. to 500° C.
 19. The methodaccording to claim 1, wherein the transformation of the steelmicrostructure takes place in a temperature range from 650° C. to 550°C.
 20. The method according to claim 3, wherein the intermediate coldsource comprises a heat exchange gas.
 21. The method according to claim20, wherein the heat exchange gas comprises a gas selected from reducinggases, inert gases and mixtures of these gases.
 22. The method accordingto claim 3, wherein the external cold source comprises a heat exchangeliquid.